The study represents a continuation of my current and past work on the organization of sensory pathways, particularly the somaesthetic and auditory in the mammalian forebrain and brain stem. One part, being carried out with light microscopic (Nauta, horseradish peroxidase and autoradiography) techniques, is an analysis of the cortical projections of the posterior group of thalamic nuclei in the monkey. It is hoped that this would contribute to our understanding of central nervous mechanisms in pain. A second part is a detailed analysis of the cellular and synaptic organization of the postcentral gyrus of the monkey using a variety of light and electron microscopic tenchiques. The object is to provide quantitative as well as qualitative data with particular emphasis on possible morphological correlates of the electrophysiologically demonstrated cell columns.